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Physicists find that misaligned carbon plates give unprecedented properties



Physicists find that misaligned carbon plates give unprecedented properties

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, with each hexagon consisting of six carbon atoms at its vertices. The UT Dallas physicists are studying the electrical properties that result from stacking two graphene layers. Photo credit: University of Texas at Dallas

A material made up of two layers of carbon, one atom thick, has attracted the attention of physicists worldwide because of its fascinating and potentially exploitable conductive properties.

Dr. Fan Zhang, assistant professor of physics at the Faculty of Science and Mathematics at the University of Texas at Dallas, and physics doctoral student Qiyue Wang published an article in June with the group of Dr. Fengnian Xia at Yale University in Natural photography that describes how the ability of twisted bilayer graphene to conduct electricity changes in response to mid-infrared light.

From one to two layers

Graphene is a single layer of carbon atoms arranged in a flat honeycomb pattern, with each hexagon consisting of six carbon atoms at its vertices. Since graphene was first isolated in 2004, its unique properties have been the subject of intense research by scientists for its potential use in advanced computers, materials, and devices.

When two layers of graphene are stacked on top of each other and a layer is rotated so that the layers are slightly out of alignment, the resulting physical configuration, called twisted bilayer graph, gives electronic properties that differ significantly from those of a single layer alone or by two aligned layers.

“Graphene has been of interest for about 1

5 years,” said Zhang. “A single layer is interesting to study, but if we have two layers, their interaction should make physics much richer and more interesting. That’s why we want to study two-layer graph systems.”

A new field is emerging

If the graphene layers are misaligned, a new periodic design is created on the web, which is called the moiré pattern. The moiré pattern is also a hexagon, but can consist of more than 10,000 carbon atoms.

“The angle at which the two graphene layers are misaligned – the twist angle – is critical to the electronic properties of the material,” said Wang. “The smaller the twist angle, the greater the moiré periodicity.”

The unusual effects of specific twist angles on electron behavior were first described in an article by Dr. Allan MacDonald, professor of physics at UT Austin, and Dr. Rafi Bistritzer proposed. Zhang witnessed the birth of this specialty as a PhD student in MacDonald’s group.

“At the time, others really weren’t paying attention to the theory, but it has now become the hottest topic in physics,” said Zhang.

In this 2011 study, MacDonald and Bistritzer predicted that the kinetic energy of the electrons could disappear in a graphene bilayer misaligned by the so-called “magic angle” of 1.1 degrees. In 2018, researchers at the Massachusetts Institute of Technology proved this theory and found that shifting two layers of graphene 1.1 degrees created a two-dimensional superconductor, a material that conducts electrical current without resistance and without loss of energy.





This animation shows what happens when two stacked graphene layers are misaligned by a small amount called the twist angle. A new periodic design is created on the web, which is referred to as a moiré pattern. The UT Dallas physicists are investigating how the twist angle affects the electronic properties of such a twisted two-layer graphene. Photo credit: University of Texas at Dallas

In an article in Science Advances from 2019, Zhang and Wang together with Dr. Jeanie Laus group at Ohio State University that twisted two-layer graphene has both superconducting and isolating states at an offset of 0.93 degrees, which significantly extends the magic angle.

“In our previous work, we saw both superconductivity and isolation. That makes the study of twisted bilayer graphene such a hot field – superconductivity. The fact that you can manipulate pure carbon to superconductivity is amazing and unprecedented,” said Wang .

New insights from UT Dallas

In his latest research in Nature Photonics, Zhang and his team at Yale investigated whether and how twisted two-layer graphene interacts with light in the mid-infrared that humans cannot see but can recognize as heat.

“Interactions between light and matter are useful in many devices – for example, to convert sunlight into electricity,” said Wang. “Almost every object, including people, emits infrared light, and this light can be captured by devices.”

Zhang is a theoretical physicist, so he and Wang wanted to find out how light in the mid-infrared can affect the conductivity of electrons in twisted two-layer graphene. Her work involved calculating light absorption based on the band structure of the moiré pattern, a concept that determines how electrons move in a material quantum mechanically.

“Graphene has been of interest for about 15 years. A single layer is interesting to study, but if we have two layers, their interaction should result in much richer and more interesting physics. So we want to study two-layer graph systems,” he says.

“There are standard methods for calculating the band structure and light absorption in a regular crystal, but this is an artificial crystal, so we had to develop a new method,” said Wang. Using the resources of the Texas Advanced Computing Center, a supercomputer facility on UT Austin’s campus, Wang calculated the band structure and showed how the material absorbs light.

The Yale group made devices and conducted experiments that showed that photosensitivity in the mid-infrared – the increase in conductivity due to the incidence of light – was unusually strong and greatest at a twist angle of 1.8 degrees. The strong light response disappeared at a twist angle of less than 0.5 degrees.

“Our theoretical results were not only in good agreement with the experimental results, but also indicated a mechanism that is essentially related to the period of the moire pattern, which is itself related to the twist angle between the two layers of graphene,” said Zhang.

Next Step

“The twist angle is clearly very important for determining the properties of twisted bilayer graphs,” added Zhang. “The question arises: Can we use this to match other two-dimensional materials to unprecedented features? Can we also combine photosensitivity and superconductivity in twisted two-layer graphene? For example, can a light shine to induce or somehow modulate superconductivity? Will be very be interesting to study. “

“This new breakthrough will potentially enable a new class of high sensitivity graphene based infrared detectors,” said Dr. Joe Qiu, program manager for solid state electronics and electromagnetics at the U.S. Army Research Office (ARO), an element of the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “These new detectors may have an impact on applications such as night vision that are critical to the U.S. Army.”


Physicists make graphene discoveries that could contribute to the development of superconductors


More information:
Bingchen Deng et al. Strong light response in the middle infrared in double-layer graphs with a small twist angle, Natural photography (2020). DOI: 10.1038 / s41566-020-0644-7

Provided by the University of Texas at Dallas



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